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Neurophysiology
Neurophysiology
Neurophysiology
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Neurophysiology is a branch of physiology and neuroscience concerned with the functions of the nervous system and their mechanisms. The term neurophysiology originates from the Greek word νεῦρον ("nerve") and physiology (which is, in turn, derived from the Greek φύσις, meaning "nature", and -λογία, meaning "knowledge").[1] Neurophysiology has applications in the prevention, diagnosis, and treatment of many neurological and psychiatric diseases.[2] Neurophysiological techniques are also used by clinical neurophysiologists to diagnose and monitor patients with neurological diseases.

The field involves all levels of nervous system function, from molecules and cells to systems and whole organisms. Areas of study include:

Experimental neurophysiologists use many techniques to study neural function. Electrophysiological techniques like electroencephalography (EEG), single cell recording, and extracellular recording of local field potentials are especially common.[3] Multi-electrode arrays on semiconductor chips can perform in vitro extracellular recording[4][5][6] and in vitro intracellular recording[7] at scale. Magnetoencephalography is sometimes used in place of EEG.[8] Immunohistochemistry, cell staining, in situ hybridisation, calcium imaging, and transmission electron microscopy are used to study cellular activity in the nervous system. Genetic engineering techniques may be used to study the impact of specific genes on neural functions. Pharmacological methods are used investigate the function of specific receptors in neurons and glia. Optogenetics and chemogenetics allow specific activation of neurons to study their functions. Functional magnetic resonance imaging and positron emission tomography can be used to measure metabolic changes in the brain.[9][8] Finally, behavioural analysis is used to understand interactions between physiology and behaviour. Contemporary neurophysiology experiments often use multiple techniques together to develop a more complete understanding of their research areas.

History

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Neurophysiology has been a subject of study since as early as 4,000 B.C.

In the early B.C. years, most studies were of different natural sedatives like alcohol and poppy plants. In 1700 B.C., the Edwin Smith surgical papyrus was written. This papyrus was crucial in understanding how the ancient Egyptians understood the nervous system. This papyrus looked at different case studies about injuries to different parts of the body, most notably the head. Beginning around 460 B.C., Hippocrates began to study epilepsy, and theorized that it had its origins in the brain. Hippocrates also theorized that the brain was involved in sensation, and that it was where intelligence was derived from. Hippocrates, as well as most ancient Greeks, believed that relaxation and a stress free environment was crucial in helping treat neurological disorders. In 280 B.C., Erasistratus of Chios theorized that there were divisions in vestibular processing in the brain, as well as deducing from observation that sensation was located there.

In 177 Galen theorized that human thought occurred in the brain, as opposed to the heart as Aristotle had theorized. The optic chiasm, which is crucial to the visual system, was discovered around 100 C.E. by Marinus. c. 1000, Al-Zahrawi, living in Iberia, began to write about different surgical treatments for neurological disorders. In 1216, the first anatomy textbook in Europe, which included a description of the brain, was written by Mondino de Luzzi. In 1402, St Mary of Bethlehem Hospital (later known as Bedlam in Britain) was the first hospital used exclusively for the mentally ill.

In 1504, Leonardo da Vinci continued his study of the human body with a wax cast of the human ventricle system. In 1536, Nicolo Massa described the effects of different diseases, such as syphilis on the nervous system. He also noticed that the ventricular cavities were filled with cerebrospinal fluid. In 1542, the term physiology was used for the first time by a French physician named Jean Fernel, to explain bodily function in relation to the brain. In 1543, Andreas Vesalius wrote De humani corporis fabrica, which revolutionized the study of anatomy. In this book, he described the pineal gland and what he believed the function was, and was able to draw the corpus striatum which is made up of the basal ganglia and the internal capsule. In 1549, Jason Pratensis published De Cerebri Morbis. This book was devoted to neurological diseases, and discussed symptoms, as well as ideas from Galen and other Greek, Roman and Arabic authors. It also looked into the anatomy and specific functions of different areas. In 1550, Andreas Vesalius worked on a case of hydrocephalus, or fluid filling the brain. In the same year, Bartolomeo Eustachi studied the optic nerve, mainly focusing on its origin in the brain. In 1564, Giulio Cesare Aranzio discovered the hippocampus, naming it such due to its shape resemblance to a sea horse.

In 1621, Robert Burton published The Anatomy of Melancholy, which looked at the loss of important characters in one's life as leading to depression.[10] In 1649, René Descartes studied the pineal gland. He mistakenly believed that it was the "soul" of the brain, and believed it was where thoughts formed. In 1658, Johann Jakob Wepfer studied a patient in which he believed that a broken blood vessel had caused apoplexy, or a stroke.

In 1749, David Hartley published Observations on Man, which focused on frame (neurology), duty (moral psychology) and expectations (spirituality) and how these integrated within one another. This text was also the first to use the English term psychology. In 1752, the Society of Friends created an asylum in Philadelphia, Pennsylvania. The asylum intended to give not only medical treatment to those mentally ill, but also provide with caretakers and comfortable living conditions. In 1755, Jean-Baptiste Le Roy began using electroconvulsive therapy for the mentally ill, a treatment still used today in specific cases. In 1760, Arne-Charles studied how different lesions in the cerebellum could affect motor movements. In 1776, Vincenzo Malacarne [it] studied the cerebellum intensely, and published a book solely based on its function and appearance.

In 1784, Félix Vicq-d'Azyr, discovered a black colored structure in the midbrain.[11] In 1791 Samuel Thomas von Sömmerring alluded to this structure, calling it the substantia nigra.[12] In the same year, Luigi Galvani described the role of electricity in nerves of dissected frogs. In 1808, Franz Joseph Gall studied and published work on phrenology. Phrenology was the faulty science of looking at head shape to determine different aspects of personality and brain function. In 1811, Julien Jean César Legallois studied respiration in animal dissection and lesions and found the center of respiration in the medulla oblongata. In the same year, Charles Bell finished work on what would later become known as the Bell–Magendie law, which compared functional differences between dorsal and ventral roots of the spinal cord. In 1822, Karl Friedrich Burdach distinguished between the lateral and medial geniculate bodies, as well as named the cingulate gyrus. In 1824, F. Magendie studied and produced the first evidence of the cerebellum's role in equilibration to complete the Bell–Magendie law. In 1838, Theodor Schwann began studying white and grey matter in the brain, and discovered the myelin sheath. These cells, which cover the axons of the neurons in the brain, are named Schwann cells after him. In 1843 Carlo Matteucci and Emil du Bois-Reymond demonstrated that nerves transmit signals electrically. In 1848, Phineas Gage, the classical neurophysiology patient, had his brain pierced by an iron tamping rod in a blasting accident. He became an excellent case study in the connection between the prefrontal cortex and behavior, decision making and consequences. In 1849, Hermann von Helmholtz studied the speed of frog nerve impulses while studying electricity in the body.

While these are not all the developments in neurophysiology before 1849, these developments were significant to the study of the brain and body.

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References

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Neurophysiology

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Neurophysiology is the branch of dedicated to the study of the functional properties of the , encompassing the electrophysiological characteristics of neurons, , and synaptic networks, as well as the mechanisms underlying neural signaling and information processing. It examines how electrochemical signals enable the coordination of sensory input, motor output, and internal regulation across the central and peripheral nervous systems. This field integrates principles from , physics, and chemistry to elucidate processes essential for , movement, , and . At its core, neurophysiology focuses on the as the primary functional unit of the , with signals transmitted via action potentials and synapses. Research in this area explores neural integration, plasticity, and systems-level functions, employing techniques from to . Clinically, neurophysiology contributes to understanding and treating neurological disorders through diagnostic tools and studies of plasticity.

Fundamentals of Neural Function

Neuron Structure and Types

are the fundamental signaling units of the , characterized by a specialized morphology that enables the reception, integration, and transmission of electrical and chemical signals. The typical consists of a cell body, or soma, which houses the nucleus and most organelles, serving as the metabolic center. Extending from the soma are dendrites, branched extensions that receive incoming signals from other , and a single that conducts outgoing signals away from the soma toward target cells. The is often enveloped by a sheath, a lipid-rich insulating layer formed by glial cells that increases the speed of signal conduction. Interruptions in the sheath, known as nodes of Ranvier, expose the axonal membrane and facilitate rapid propagation of electrical impulses. At its distal end, the terminates in synaptic terminals, or boutons, which form connections with other cells. Neurons are classified by function into sensory neurons, which transmit information from sensory receptors to the ; motor neurons, which carry signals from the to effectors like muscles; and , which integrate signals within the . Morphologically, neurons are categorized as unipolar, with a single process that bifurcates; bipolar, featuring one and one ; or multipolar, possessing multiple dendrites and a single , the most common type in the . Specific examples include pyramidal cells, multipolar neurons with a triangular soma and apical dendrite prominent in the , and granule cells, small multipolar neurons found in the and . Structural adaptations enhance neuronal efficiency. Dendritic spines are small, protrusive structures on dendrites that increase surface area for synaptic contacts and compartmentalize signaling molecules. maintains neuronal polarity through motor proteins: drives anterograde movement of vesicles and organelles toward the , while facilitates retrograde transport back to the soma. Glial cells provide essential structural support to neurons. Astrocytes, star-shaped glia, supply nutrients like glucose and lactate to neurons via their extensive processes and maintain the extracellular environment. Oligodendrocytes in the central nervous system produce the myelin sheath, insulating axons to support rapid conduction, while Schwann cells perform this role in the peripheral nervous system.

Resting Membrane Potential and Ion Dynamics

The resting membrane potential (RMP) of a is the electrical potential difference across its plasma membrane when the cell is not actively transmitting signals, typically ranging from -60 to -80 mV, with the interior negative relative to the exterior. This potential arises from unequal distributions of ions across the membrane, primarily (K⁺), sodium (Na⁺), and (Cl⁻), combined with the membrane's selective permeability to these ions. Intracellular K⁺ concentration is approximately 140 mM, while extracellular is about 5 mM; extracellular Na⁺ is around 145 mM, and intracellular about 15 mM; intracellular Cl⁻ is roughly 7 mM, and extracellular about 110 mM. These steep concentration gradients are actively maintained by the Na⁺/K⁺-ATPase pump, which hydrolyzes ATP to transport 3 Na⁺ ions out of the cell and 2 K⁺ ions in, creating a net electrogenic effect that contributes slightly to the negative RMP. The pump was first identified by in 1957. The equilibrium potential for a single ion species, known as the Nernst potential, represents the membrane voltage at which the chemical and electrical driving forces on that ion balance, resulting in zero net flux. It is calculated using the Nernst equation: Eion=RTzFln⁡([ion]out[ion]in)E_{\text{ion}} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{out}}}{[\text{ion}]_{\text{in}}} \right) where RR is the gas constant, TT is temperature in Kelvin, zz is the ion's valence, and FF is Faraday's constant. This equation, derived by Walther Nernst in 1889, applies to ions like K⁺ (z=+1z = +1), yielding EK+≈−90E_{\text{K}^+} \approx -90 mV under typical neuronal conditions, close to the RMP and reflecting high K⁺ permeability. For Na⁺, ENa+≈+60E_{\text{Na}^+} \approx +60 mV due to its outward concentration gradient; for Cl⁻ (z=−1z = -1), ECl−≈−70E_{\text{Cl}^-} \approx -70 mV, often near the RMP in some neurons. In reality, the RMP is a weighted of these individual equilibrium potentials, determined by the membrane's relative permeabilities to multiple ions, as described by the Goldman-Hodgkin-Katz (GHK) voltage . The GHK is: Vm=RTFln⁡(PK[K+]out+PNa[Na+]out+PCl[Cl−]inPK[K+]in+PNa[Na+]in+PCl[Cl−]out)V_m = \frac{RT}{F} \ln \left( \frac{P_{\text{K}}[\text{K}^+]_{\text{out}} + P_{\text{Na}}[\text{Na}^+]_{\text{out}} + P_{\text{Cl}}[\text{Cl}^-]_{\text{in}}}{P_{\text{K}}[\text{K}^+]_{\text{in}} + P_{\text{Na}}[\text{Na}^+]_{\text{in}} + P_{\text{Cl}}[\text{Cl}^-]_{\text{out}}} \right) where PP denotes permeability coefficients. First formulated by David E. Goldman in 1943 and refined by Alan Hodgkin and Bernard Katz in 1949, this equation accounts for the steady-state potential under constant field assumptions. In neurons, the membrane is predominantly permeable to K⁺ (PK≫PNa,PClP_{\text{K}} \gg P_{\text{Na}}, P_{\text{Cl}}), so the RMP is pulled toward EK+E_{\text{K}^+}, typically stabilizing at -70 mV. This selective permeability is largely due to leak channels, which are constitutively open ion channels that allow passive diffusion down electrochemical gradients, with K⁺ leak channels (e.g., inward rectifiers) dominating at rest. The Na⁺/K⁺ pump counters the slow dissipation of gradients through these leaks, ensuring long-term stability of the RMP essential for neuronal excitability. Without the pump's activity, the RMP would depolarize over time as ions equilibrate.

Action Potential Generation

Action potential generation is the process by which a converts a graded depolarizing stimulus into a rapid, self-propagating electrical signal that transmits information along the . This transient reversal of the from negative (resting around -70 mV) to positive values occurs when the potential reaches a threshold, typically -55 mV in many s, activating voltage-gated ion channels in an all-or-none manner. The seminal quantitative description of this mechanism came from voltage-clamp experiments on the , revealing the roles of sodium and potassium conductances in initiating and shaping the signal. The all-or-none principle governs initiation: once threshold is exceeded, the response fires at full amplitude regardless of further increases in stimulus intensity, ensuring reliable signaling without gradation. This property, first demonstrated for isolated fibers, prevents of subthreshold events into partial responses and was key to early electrophysiological studies. The threshold dynamics reflect a balance where regenerative Na+ entry overcomes passive leak, but small variations in channel density or ion gradients can shift it slightly across types. The phases of the action potential begin with , where voltage-gated Na+ channels open rapidly upon reaching threshold, permitting Na+ influx that drives the potential toward the Na+ equilibrium (~+55 mV), peaking at +30 to +40 mV within 1 ms. This loop is terminated as Na+ channels inactivate and delayed rectifier K+ channels activate, leading to via K+ efflux that restores negativity. A subsequent after-hyperpolarization often follows, as lingering K+ conductance pulls the potential below resting levels before returning to equilibrium. These phases, lasting 2-4 ms total, were precisely characterized through the Hodgkin-Huxley experiments, which isolated ionic currents under controlled voltages. The biophysical basis is encapsulated in the Hodgkin-Huxley model, a set of nonlinear differential equations derived from fitting experimental data to predict behavior. The core for voltage change is dVdt=−(INa+IK+Ileak+Istim)Cm\frac{dV}{dt} = -\frac{(I_\mathrm{Na} + I_\mathrm{K} + I_\mathrm{leak} + I_\mathrm{stim})}{C_m} where VV is , CmC_m is (~1 μF/cm²), IstimI_\mathrm{stim} is applied current, and ionic currents are ohmic: INa=gˉNa m3h (V−ENa),IK=gˉK n4 (V−EK),Ileak=gleak (V−Eleak).I_\mathrm{Na} = \bar{g}_\mathrm{Na} \, m^3 h \, (V - E_\mathrm{Na}), \quad I_\mathrm{K} = \bar{g}_\mathrm{K} \, n^4 \, (V - E_\mathrm{K}), \quad I_\mathrm{leak} = g_\mathrm{leak} \, (V - E_\mathrm{leak}). Here, gˉNa\bar{g}_\mathrm{Na} (120 mS/cm²) and gˉK\bar{g}_\mathrm{K} (36 mS/cm²) are maximum conductances, ENaE_\mathrm{Na} (+50 mV) and EKE_\mathrm{K} (-77 mV) are reversal potentials, and gating variables mm (Na+ activation), hh (Na+ inactivation), and nn (K+ activation) evolve via dxdt=αx(V)(1−x)−βx(V)x(x=m,h,n),\frac{dx}{dt} = \alpha_x(V) (1 - x) - \beta_x(V) x \quad (x = m, h, n), with rate functions αx,βx\alpha_x, \beta_x empirically determined from voltage-clamp traces to capture the voltage- and time-dependent channel kinetics. This model accurately reproduces shape, threshold (~-52 mV in ), and propagation when solved numerically. Following generation, refractory periods limit firing rate and ensure unidirectional propagation. The absolute refractory period (1-2 ms) coincides with and early , during which Na+ channel inactivation prevents re-excitation despite strong stimuli. The relative refractory period (2-4 ms) overlaps with hyperpolarization, requiring suprathreshold input due to elevated K+ conductance and partial Na+ recovery. These periods, intrinsic to the gating dynamics in the Hodgkin-Huxley framework, were evident in the model's simulations of successive stimuli. Once initiated at the axon hillock, the action potential propagates without amplitude loss via local currents that depolarize adjacent membrane. In unmyelinated axons, continuous conduction spreads the active zone progressively, with velocity scaling as the square root of diameter (v∝dv \propto \sqrt{d}
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